The symptoms you describe are very, very unlikely to have been caused by any sort of change in soil pH. The rapid collapse of azalea flowers, assuming the plants have had adequate water, is characteristic of ovulinia petal blight, a fungus infection that spreads rapidly, especially in damp weather. It usually begins with spots appearing on the flowers, then collapse of the flowers into a slimy mass. This is really a guess, but hopefully points you in the right direction. Since ovulinia effects only the flowers, the leaf drop last summer would seem to have been caused by something else - drought, insects such as lace bugs, fungal attack on the roots, or any number of other causes. As I am uncertain where you are located, I would suggest consulting with whatever agency deals with agriculture and horticulture for an examination of your plants and a diagnosis of their problems.

There haven't been many dry days this spring in Toronto. So far only these 2 azaleas have been affected. Now, I worry about rhododendrons and several other azaleas.